Who vs Whom.
Who is used as a subject and whom as an object. The examples below illustrate correct usage.
Give the award to whoever will benefit most. [Subjective case: whoever is the subject of will benefit.]
Whom did you consult? [Objective case: whom is the object of consult.]
Who was the consultant on this case? [Subjective case: who is the subject of the sentence.]
He is one of the patients whom Dr Rundle is treating. [Objective case: whom is the object of is treating.]
He is one of the patients who are receiving the placebo. [Subjective case: who is the subject of are receiving.]
That vs Which.
Relative pronouns may be used in subordinate clauses to refer to previous nouns. The word that introduces a restrictive clause, one that is essential to the meaning of the noun it describes. The word which introduces a nonrestrictive clause, one that adds more information but is not essential to the meaning. Clauses that begin with which are preceded by commas. Two examples of correct usage follow.
A study on the impact of depression on US labor costs was published in the 2003 JAMA theme issue on depression, which contains articles on a range of similar topics. [Nonrestrictive; there was only one theme issue on depression in 2003.]
The issue of JAMA that contained the article on the impact of depression on US labor costs was the 2003 depression theme issue. [Restrictive; there are thousands of issues of JAMA.]
Following are examples of ambiguous or incorrect usage that highlight this grammatical problem.
The high prevalence of antibodies to the 3 Bartonella species, which were examined in the present study, indicates that health care workers should be alert to possible infection with any of these organisms when treating intravenous drug users. [There are more than 3 species of Bartonella. Hence, the correct form here would be “… the 3 Bartonella species that were examined.…”]
Many reports have been based on series of patients from urology practices that may not fully reflect the entire spectrum of illness. [Do the patients or the practices not fully reflect the entire spectrum of illness? Also, do the reports involve all or only some urology practices?]
Many reports have been based on patients in urology practices, which may not fully reflect the entire spectrum of illness. [Urology practices in general do not capture the range of the disease.]
Many reports have been based on data from urology practices that may not fully reflect the entire spectrum of illness. [Some particular urology practices do not capture the range of the disease, but others might.]
This morning he revealed evidence that calls the study’s integrity into question has been verified.
This morning he revealed that evidence that calls the study’s integrity into question has been verified.
The addition of that after revealed frees the reader from backtracking to uncover the meaning of the sentence above. The use of that to introduce a clause is particularly helpful when the second verb appears long after the first has been introduced (above, the interval between revealed and has been verified).